I-III-VI₂ ternary semiconductor nanocrystals, such as CuInS₂ and AgInS₂, exhibiting the quantum size effect have attracted much attention for the application to solar energy conversion systems because of their strong absorption coefficient and low toxicity. The optical properties of these particles are tunable by controlling the particle size. Recently we have successfully prepared anisotropic-shaped nanocrystals of ZnS-AgInS₂ solid solution ((AgIn)_xZn_2(1-x)S₂, ZAIS). Their photocatalytic H₂ evolution activity could be controlled by the chemical composition as well as by the particle size,⁽¹⁾ and increased with particle morphology in the order of rice < sphere < rod.⁽²⁾ On the other hand, the formation of type II heterojunction between different semiconductors was reported to be another strategy to enhance the photocatalytic activity of composite particles due to the effective charge separation of photogenerated electrons and holes at the heterojunction. In this study, we synthesize dumbbell-shaped nanocrystals composed of ZnS-AgInS₂ solid solution (ZAIS) via epitaxial crystal growth in the solution phase, in which the heterojunction forms between rod- and rice-shaped parts in the nanocrystals. The photocatalytic activity of resulting nanocrystals is investigated for H₂ evolution as a model reaction.

Rod-shaped ZAIS nanocrystals with sizes of 4.1 × 23 nm as a precursor were prepared by the previously reported method.⁽²⁾ These nanocrystals were heat-treated at 170 °C for 8 min in an oleylamine/1-dodecanethiol mixture solution containing AgCH₃COO, In(CH₃COO)₃, and thiourea. Thus-obtained mixture nanocrystals were isolated from the resulting solution by adding methanol as a non-solvent. Dumbbell-shaped nanocrystals were separated from rice-shaped ones as a by-product with use of a size-selective precipitation technique. The photocatalytic activity for H₂ evolution was investigated by the irradiation of dumbbell- and rice-shaped ZAIS nanocrystals in a mixture solution of water/2-propanol (1:1) containing Na₂S as a hole scavenger with a Xe lamp (λ > 350 nm).

The XRD analysis revealed that these ZAIS particles had a wurtzite crystal structure. With TEM measurements, we found that rice-shaped crystals with sizes of 5.6 × 11 nm were epitaxially grown on both the tips of rod-shaped ZAIS nanocrystals, resulting in the formation of dumbbell-shaped nanocrystals. The energy gap of dumbbell-shaped nanocrystals was determined to 1.9 eV from the absorption onset, being equal to that of freely dispersed nanocrystals with rice shape but lower than that of original rod-shaped ones, 2.8 eV. These suggested that the Zn content in rod-shaped parts of dumbbell-shaped nanocrystals was higher than that in rice-shaped parts of the same particles. By estimating the electronic energy structure of dumbbell-shaped ZAIS nanocrystals from those of corresponding rod- and rice-shaped nanocrystals, the heterojunction of type II structure was expected to form at the interface between rod- and rice-shaped parts in a dumbbell nanocrystal.

ZAIS nanocrystals were dispersed in water/2-propanol solution and irradiated with a Xe lamp light. The H₂ evolution was observed, the amount of which increased linearly with elapse of irradiation time. The H₂ evolution rate of dumbbell nanocrystals was about four times larger than that of rice-shaped ones. These results indicated that the ZAIS nanocrystals worked as a photocatalyst and then the type II heterojunction in dumbbell-shaped nanocrystals induced the effective charge separation of photogenerated electrons and holes.

Reference (1) T. Torimoto et al., J. Phys. Chem. C., 2015, 119, 24740-24749. (2) T. Torimoto et al., ACS Appl. Mater. Interfaces 2016, 8, 27151-27161.